1. Field of the Invention
The present invention relates to a zoom lens system suitable for a photo-taking camera, video and electronic still cameras and the like.
2. Description of the Related Art
As a zoom lens system having an angle-of-view of about 75xc2x0 at the short focal length extremity and a zoom ratio of more than 3.7, the ones, for example, disclosed in Japanese Unexamined Patent Publication (JPUEPP) No. Hei-6-130299, JPUEPP No. Hei-4-149402, and U.S. Pat. No. 5,815,321 are known in the art. However, the zoom lens system taught in JPUEPP No. Hei-6-130299 has required high production costs, since (i) the number of lens elements constituting the zoom lens system is relatively larger, i.e., fourteen lens elements, and (ii) an F-number is also relatively larger, i.e., about 5 to 8. If this zoom lens system is used in a single lens reflex (SLR) camera, focusing becomes difficult due to a darker image, i.e., a larger F-number.
In the zoom lens systems taught in JPUEPP Nos. Hei-4-149402 and U.S. Pat. No. 5,815,321, the number of lens elements are smaller; however, higher production costs are still required since aspherical surfaces are employed in order to enhance optical performance.
At least the following process and devices are known in the art to form aspherical surfaces:
(i) a machining process through which an aspherical surface is directly formed on a glass lens element;
(ii) the so-called hybrid molding dies through which a thin resin layer is bonded on a glass lens element; and
(iii) the molding dies through which an entire lens element with an aspherical surface is formed.
It is noted that the machining process needs machinery with high performance, and the hybrid molding dies and the molding dies need to be formed in the shape of an aspherical surface. Any one of these process and devices therefore requires higher costs.
Furthermore, in the inspection process for lens elements, spherical surfaces can be inspected with a Newton gauge which has high precision, and is a simple and convenient inspection means; however, aspherical surfaces need specific inspection means in accordance with the surface shapes thereof, which is generally time consuming and costly.
It is an object of the present invention to provide a four-lens-group zoom lens system, with a small number of lens elements, which has high optical performance and is inexpensive.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, the first through fourth lens groups move toward the object so that the distance between the first and the second lens groups becomes longer, the distance between the second and the third lens groups becomes shorter, and the distance between the third and the fourth lens groups becomes shorter. The third lens group includes a positive 3-1st lens element, a positive 3-2nd lens element having a large-curvature convex surface facing toward the object, and a negative 3-3rd lens element having a large-curvature concave surface facing toward the object, in this order from the object. The zoom lens system satisfies the following condition:
xe2x88x921.8 less than Ør3-3-1/Øw less than xe2x88x921.1..xe2x80x83xe2x80x83.(1)
wherein
Ør3-3-1 designates the surface power of the object-side surface of the negative 3-3rd lens element of the third lens group; and
Øw designates the power of the entire lens system at the short focal length extremity.
The fourth lens group preferably includes a positive 4-1st lens element having a large-curvature convex surface facing toward the image, a positive 4-2nd lens element, and a negative 4-3rd lens element having a large-curvature concave surface facing toward the object, in this order from the object. The zoom lens system preferably satisfies one of, or both condition (2) and condition (3):
xe2x88x921.8 less than Ø4air/Øw less than xe2x88x921.2..xe2x80x83xe2x80x83.(2)
SF4air less than xe2x88x921.1.xe2x80x83xe2x80x83.(3)
wherein
Ø4air=Ø4-2-2+Ø4-3-1xe2x88x92d4-4xc3x97Ø4-2-2xc3x97Ø4-3-1
d4-4 designates the distance between the positive 4-2nd lens element and the negative 4-3rd lens element;
Ø4-2-2=(1xe2x88x92n4-2)/r4-2-2
Ø4-3-1=(n4-3xe2x88x921)/r4-3-1
n4-2 designates the refractive index of the positive 4-2nd lens element;
n4-3 designates the refractive index of the negative 4-3rd lens element;
SF4air=(r4-3-1+r4-2-2)/(r4-3-1xe2x88x92r4-2-2);
r4-2-2 designates the radius of curvature of the image-side surface of the positive 4-2nd lens element; and
r4-3-1 designates the radius of curvature of the object-side surface of the negative 4-3rd lens element.
In the zoom lens system according to the present invention, upon focusing from an object at an infinite photographing distance (i.e, a camera-to-object distance) to an object at the closest photographing distance, the second lens group is preferably moved toward the object. According to this arrangement, optical performance for closer photographing distances can suitably be improved.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-144820 (filed on May 25, 1999) which is expressly incorporated herein by reference in its entirety.